Turbochargers which compress intake air into the engine using the energy of the exhaust gas to increase engine output without altering the engine displacement are widely used. The exhaust energy is recovered by a turbine provided in an exhaust passage, and is transmitted by means of a rotating shaft fixed at one end of the turbine, to a compressor impeller provided in an intake passage so as to rotate the impeller. The impeller rotates at from several tens of thousand to several hundred thousand rpm depending on the engine operation, and compresses the air passing through the intake passage to the engine.
To improve the response of the above type of turbocharger, that is response in relation to accelerator operation, a recent widely accepted practice has been to support the rotating shaft by means of rolling bearings. FIG. 1 shows an example of a support construction for a rotating shaft of this type.
A pair of outer spacers 2 are loosely fitted inside a housing 1, and angular type ball bearings 4 are provided between the inner peripheral faces of the outer spacers 2 and an outer peripheral face of a rotating shaft 3, respectively. The ball bearings 4 each comprise an outer ring 6 having an outer raceway 5 on an inner peripheral face thereof, an inner ring 8 having an inner raceway 7 on an outer peripheral face thereof, a plurality of balls 9 provided so as to be freely rotatable between the outer raceway 5 and the inner raceway 7, and a retainer 10 for rotatably retaining the plurality of balls 9. The outer rings 6 of the respective ball bearings 4 are fixedly fitted inside the outer spacers 2, while the inner rings 8 are fixedly fitted around the rotating shaft 3, so that the rotating shaft 3 is supported so as to be freely rotatable inside the housing 1.
An inner spacer 11 is provided to keep the pair of inner rings 8 suitably spaced from each other, while a compression spring 12 is provided for applying a pre-load to the back-to-back pair of angular type ball bearings 4.
With the above type of construction, the ball bearings 4 for supporting the rotating shaft 3 require a high degree of heat resistance. In particular, the ball bearing 4 provided on the side of the turbine which absorbs the exhaust energy, comes under a strong influence from the heat of the high temperature exhaust gas (maximum temperature of approximately 950.degree. C.), and thus requires a much higher degree of heat resistance. In view of this, heretofore U.S. Pat. No. 5,028,150, and Publication of Unexamined Japanese Patent Application KOKAI No. H2-70923 for example, disclose inventions related to turbocharger bearings wherein the inner ring which is exposed to a particularly high temperature is made from a heat resistant material such as M50, AISI 440C.
On the other hand, Publications of Japanese Unexamined Utility Model KOKAI Nos. H2-54925, H3-88023 and H4-95125 and Publications of Unexamined Japanese Patent Application KOKAI Nos. H3-96716, H3-117722, H3-188127, H4-29617, disclose inventions wherein the heat resistance of the retainer is improved through manufacture of the retainer using a heat resistant resin such as polyimide resin, Teflon, PTFE, etc. The retainer of H4-95125 is used in a turbocharger bearing for automobiles. Moreover, in the disclosure of Publication of Unexamined Japanese Patent Application KOKAI No. H1-159419, the balls 9 between the steel outer and inner rings of the ball bearing 4 are made from a ceramic. KOKAI No. H2-54925 above also has a ceramic rolling member. The ball bearings disclosed in the above publications which do not have retainers are so-called full compliment ball bearings.
With the turbo charger bearings heretofore known, only one part of the components of the ball bearing is improved, and there is no improvement in the interconnection of components. Consequently depending on operating conditions, there can be situations wherein heat resistance is inadequate, and sufficient endurance cannot be obtained, or turbo charger response cannot be improved.
Publications of Unexamined Japanese Patent Application KOKAI Nos. S60-208626, H1-220718 and H1-220719 disclose inventions related to rolling bearings comprising outer and inner rings having both heat resistance and corrosion resistance, and a rolling body made of ceramic. However, the object of these inventions is a rolling bearing for supporting a roller during hot dipping, and the bearings as disclosed, being without a retainer and the like, are not applicable for use as turbo charger ball bearings.
That is to say, in the case of rolling bearings such as the ball bearing disclosed in the beforementioned Publication of Unexamined Japanese patent Application KOKAI No. H1-159419 wherein a retainer for retaining the rolling body is not provided, the adjacent pairs of rolling bodies rub against each other with rotation of the shaft. Consequently undesirable resistance to rotation of the shaft is increased, and when used for supporting a rotating shaft of a turbo charger, response is undesirably impaired.
On the other hand, Publication of Unexamined Japanese Utility Model Application KOKAI No. H2-54925, and Technical Bulletin No. 92-1545 of Japan Institute of Invention and Innovation (Hatsumei Kyokai), disclose a construction wherein a retainer for a turbo charger ball bearing is made of a heat resistant synthetic resin and is guided by an outer ring. If the retainer is made from a synthetic resin having a low specific gravity, the response can be improved due to the reduction in inertial mass, while if the retainer is guided by the outer ring, oscillation of the retainer in a radial direction can be prevented and vibration during running can be minimized.
However, simply making the retainer from a synthetic resin and having the retainer guided by the outer ring does not result in a sufficiently satisfactory performance. That is to say, since the coefficient of linear expansion of synthetic resin greater than that of metals such as brass, steel and the like, then if the relationship between the outer diameter of the synthetic resin retainer and the diameter of the inner peripheral face of the outer ring acting as a guide for the synthetic resin retainer is not suitably determined, the resistance of the retainer to rotation becomes large or rattling of the retainer becomes excessive.
For example, if the outer peripheral dimension of the synthetic resin type retainer is close to the diameter of the inner peripheral surface of the outer ring, so that the gap between the outer peripheral face of the retainer and the inner peripheral face of the outer ring will be small, the outer peripheral face of the retainer and the inner peripheral face of the outer ring become very close together or in contact with each other with an increase in temperature during operation of the turbo charger.
if the outer peripheral face and inner peripheral face are very close together, it is difficult for the lubricating oil existing around the balls 9 to discharge to an outer portion; so that the amount of lubricating oil existing around the balls becomes more than required. As a result, the churning resistance of the lubricating oil due to the balls 9 is increased, the drive resistance of the ball bearings is increased, and the response of the turbo charger incorporating the ball bearings is impaired.
Moreover, with contact of the outer peripheral face of the retainer and the inner peripheral face of the outer ring, resistance to rotation of the retainer becomes extremely large, and not only is the response of the turbo charger severely impaired but also, depending on the situation, there is a possibility of seizure of the ball bearings.
On the other hand, by making the outer diametric dimension of the synthetic resin retainer smaller compared to the diameter of the inner peripheral face of the outer ring, then the clearance or gap between the outer peripheral face of the retainer and the inner peripheral face of the outer ring will be larger so that the retainer becomes susceptible to vibration during running. More specifically, it is difficult to make the mass of the synthetic resin retainer perfectly even around the circumferential direction and although the unevenness may be very slight, an unbalance in the mass around the peripheral direction cannot be avoided. As a result, if the clearance or gap large, then because of a free radial change in position of the retainer itself, the retainer vibrates when the retainer rotates at the high speed with running of the turbo charger.
Such unbalanced vibration of the synthetic resin retainer produces a harsh noise, and in extreme cases wear of the retainer and the possibility of failure.
The ball bearing for turbo charger of the present invention was conceived in view of the above situation.